the invention relates to a direct conversion receiver in which an FM signal is frequency down-converted in mixers to quadrature related I and Q signals at baseband frequencies, which signals are applied to demodulating means in which the modulation in the FM signal is recovered by obtaining the differential with respect to time of the arctan of the I and Q signals. A typical prior-art FM detector using a quadrature detector circuit, an arctan A/D converter, and a derivative circuit is shown in U.S. Pat. No. 4,603,300.
An advantage of a direct conversion receiver (or zero-IF receiver) architecture in which the incoming RF signal is mixed directly down to baseband over a superheterodyne receiver architecture is that all the channel amplification and filtering can be performed at baseband, which makes the direct conversion receiver suitable for integration. However, one of the problems with a direct conversion receiver is that a d.c. offset voltage will appear at the mixer output in addition to the wanted baseband signal. This d.c. offset can be much greater in amplitude than the wanted signal and so must be removed. It can be eliminated by a.c. coupling the mixer output, but this will also remove the wanted d.c. component produced by the down-converted carrier and in addition will introduce phase distortion in the baseband quadrature related signals. These two effects caused by a.c. coupling make it difficult to FM demodulate the incoming signal. The FM demodulation is performed by calculating the arctan of the I and Q channel signals. This unwanted d.c. component arises from d.c. offsets within the mixer and from the mixing down to d.c. of local oscillator leakage from the local oscillator to the r.f. port of mixer. A good double balanced diode ring mixer may have a LO-RF isolation of typically 40 dB and a d.c. offset of about 1 mV. This d.c. term can be much greater in amplitude than the wanted signal and would cause the post-mixer amplifiers and filters to saturate and so it must be removed.
The unwanted d.c. component is not constant, but will vary with local oscillator frequency and power level, and so it cannot be nulled out easily. It can be eliminated by a.c. coupling the mixer output, but this also removes the wanted d.c. component produced by the down-converted carrier. This wanted d.c. component depends on the phase of the incoming carrier with respect to that of the local oscillator. The consequence of removing the wanted d.c. component is that the values of the I and Q channel signals, representing the phase of the r.f. signal, will be incorrect after a.c. coupling. Thus frequency detection methods which are based on phase detection followed by a differentiation will fail, since an incorrect phase is detected.
A further problem is caused by the fact that the a.c. coupling has to be carried out in the analog part of the circuit, before analog to digital (A/D) conversion, because the A/D converters have a limited dynamic range. Therefore, a filter which carries out the a.c. coupling will be an analog high pass filter. Such a type of filter will cause a frequency dependent phase shift in the I and Q channel signals, which will also distort the output signal severely if frequency detection, based on phase detection followed by differentiation, is used.